原式=lim [3^n/4^(n+1)+1/4]/[3^(n+1)/4^(n+1)+1] 上下同时除以4^(n+1)
=lim [1/4*(3/4)^n+1/4]/[(3/4)^(n+1)+1]
=(0+1/4)/(0+1)
=1/4